Running dnf to update and install additional packages every time
the job is executed slows down the CI pipeline. Avoid this by
using another custom images for JS source checks.
In addition to the js shell we use for the existing syntax check,
also include eslint for future jobs and some extension-specific
tooling to make the image more useful to extension authors.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/408
